Some microstructures of experimentally deformed argillaceous sediments

Abstract

Soft argillaceous sediments have been experimentally deformed and the resulting microstructures examined in thin section. Specimens of various natural clays, ball clay, kaolinite, etc., with saline water contents up to 40% have been subjected to (i) triaxial compression at strain rates between 10 −3/sec. and 10 −7/sec. and confining pressures up to 300 bars, (ii) direct and distributed shear at similar strain rates and with normal stress up to 14 bars. In general the materials are weak and ductile but slip is involved in most of the wide range of microstructures that is produced. None of the effects is wholly pervasive. Deformation at failure is chiefly accomplished by ductile shear along inclined, discrete, planar zones. ‘Layer boundary crenulations’ consist of broad, tapering crenulations which form at high angles to the junction with a layer of contrasting lithology. The structure has been recognized in a number of rocks which are thought to have undergone some movement while still soft. Crenulations of the primary settling fabric are common but vary greatly in spacing, width and sharpness. At one extreme the crenulations grade into discrete kink-bands and at the other produce closely-spaced highly-asymmetric ‘creases’ which may have important implications for the initiation of slaty cleavage. The microstructures may provide new guides for recognizing pre-lithification movements in sedimentary rocks.